1. Field of the Invention
The invention relates generally to methods of and apparatus for transferring materials. More particularly, the invention relates to methods of and apparatus for supplying a paving machine with paving material, such as an asphaltic material.
2. Discussion of the Prior Art
In the art of paving large surfaces such as airport runways, automobile parking lots and various types of roadways with asphaltic aggregate materials, paving machines of the floating screed type have been perfected to lay a path of a typical width of a roadway, typically of between ten to twenty feet in a continuous operation. In fact, the advance of the paver at a preferably constant rate is desirable. Stopping the paver in its operation for any reason, including loading material into its supply hopper, is likely to cause the screed to settle from its normally floating position into the newly laid width of material to result in a grade or density variation and in possibly permanent pavement imperfections. It is therefore desirable to supply the paver with paving materials, typically asphaltic aggregate materials, to enable the paver to proceed at a constant rate in its paving operation. Trucks are typically used to haul loads of the paving material from a mixing site to the current paving site.
A known method to supply the materials to the paver is by allowing the trucks to dump the material to form a windrow along the centerline of the strip of pavement to be laid down by the paver. A windrow elevating machine may be mounted to the front of the paver to be pushed along by the paver. As the paver advances, the elevating machine scoops up the material from the base grade of the roadway and transfers the material by means of a chain conveyor, also referred to as slat conveyor, to the feed hopper of the paver. The windrow supply method entails some shortcomings. Obviously, the amount of material in the windrow needs to equal the amount of material to be laid down by the paving operation. Consequently, the material dump needs to be controlled. Proper spacing between successive truck loads depends on the amount of material each of the trucks is carrying. An excess of material quickly shows up in a filled feed hopper of the paver. Removing material ahead of the paver requires time and possibly results in an undesirable stoppage of the paving operation. On the other hand, a shortage of material may also result in a stoppage of the paving operation, the windrow ahead of the paver making it now more difficult to supply additional material to the paver. Other difficulties which may be experienced is that base grade material may be scooped up by the windrow elevating machine, possibly resulting in a defect in the pavement. Yet, many experienced paving contractors consider the windrow material deposit method to be an overall cost effective paving method.
An alternative to a windrow material deposit method for paving an asphalt roadway is a direct truck dump supply method. According to this alternative method, the truck is backed toward the paver and comes to rest against a truck roll at the front edge of the feed hopper of the paver. The paver then pushes the truck along as the truck unloads the material into the feed hopper of the paver. The typical height of a truck bed and a lack of overhang of its end prevent a truck dump from filling the feed hopper of the paver to a maximum capacity. A feed hopper with a maximum capacity of approximately fifteen tons of asphaltic aggregate material may, for example, only be loaded to a capacity of from 5 to 6 tons of material from an unloading truck. Assuming a fast yet reasonable paving speed of 60 feet per minute, and assuming that a strip three inches thick and fourteen feet wide is paved, an estimated fourteen tons of material per minute is required. The required supply rate allows less than 30 seconds for a truck exchange, a condition which is hardly achievable. A windrow elevating machine, in comparison, has a capability of scooping up and loading a maximum of 2000 tons of material per hour into the feed hopper of the paver, resulting in a continued preference for using the windrow method of providing material whenever possible.
It has been sought to overcome disadvantages of windrow supply methods and direct truck dump methods by a self-propelled storage vehicle of paving material. The vehicle has a hopper with a storage capacity approximately equal to the capacity of one of the material supply trucks. A high capacity slat conveyor similar to the conveyor used by the above-discussed windrow elevating machine allows the truck to dump the material and transfer the material into the hopper of the storage vehicle. The material is then transferable from the hopper to the feed hopper of the paver by a second conventionally pivotable conveyor. Because the second conveyor is capable of elevating the material from the hopper of the storage vehicle to a discharge point above the feed hopper of the paver, the feed hopper of the paver can be filled to the maximum capacity of approximately 15 tons of paving material.
Though each of the discussed methods and associated apparatus for supplying paving material to a paver may solve some of the problems associated with one of the other methods, each method itself has limitations which may not be encountered when using one of the other methods or apparatus. One of the limitations of the added storage vehicle is an added cost factor resulting from the additional, substantial apparatus and the additional personnel needed to operate and maneuver the storage vehicle. Another factor introduced by the storage vehicle is a recognized problem of material separation. Aggregate materials are known to separate into more and less coarse aggregates as the material is transferred from one storage medium to another. A slight degree of separation may be acceptable as unavoidable but is nevertheless undesirable. It is therefore advantageous to minimize separation and reclassification of aggregate materials.